The present invention relates to a method for transferring a thin layer of monocrystalline material from a first monocrystalline substrate to a second substrate, the transferred thin layer being initially an upper portion of the first substrate. In this method the lower portion of the first substrate remains and can be used to produce additional thin layers for subsequent transfer. For many applications, especially in the area of semiconductors, monocrystalline material is required in order to fabricate high performance microelectronic or optoelectronic devices. In many such applications it is only required that a thin surface layer, on the order of 10 nanommeters to a several micrometers, consists of the monocrystalline material while the rest of the body can consist of any appropriate substrate. Only if the substrate upon which the epitaxial layer formed is both monocrystalline and has a lattice constant which is close to that of the surface on which it is formed can such an epitaxial layer be grown by well established epitaxial methods. In the invention disclosed here the second substrate can have either a very different lattice constant or can be polycrystalline or even amorphous or can be monocrystalline and covered with an amorphous or polycrystalline layer. In previous inventions the fabrication of a layer of monocrystalline material on a second substrate is not possible by layer transfer if the thermal expansion coefficients of the two substrates do not closely match. The transfer of the thin monocrystalline layer from an appropriate monocrystalline first substrate with the same lattice constant as the monocrystalline layer to a second substrate, whose lattice constant is different from that of the first substrate, by means of this bonding approach together with the subsequent removal of the first substrate minus the thin transferred layer avoids the need for epitaxial growth on the second substrate. The monocrystalline first substrate may consist of the same material as the monocrystalline layer to be transferred or it may consist of a different monocrystalline material but still with nearly the same lattice constant. The bonding may either be anodic bonding, in which case the second substrate can be a glass with a sufficiently high electrical conductivity, or the bonding may be direct wafer bonding as disclosed by Stengl and Goesele in U.S. Pat. No. 4,883,215 or the bonding can be by still other methods.
In silicon-on-insulator material, the thin monocrystalline layer consists of monocrystalline silicon and the second substrate consists of an oxidized silicon wafer in which the silicon wafer is covered by an amorphous oxide layer. In this case the monocrystalline silicon layer is initially a part of the first substrate, which consists of a monocrystalline silicon wafer usually covered by a thin oxide layer or a purposely grown amorphous oxide layer which is bonded to the second substrate, which also consists of a monocrystalline silicon wafer covered by an oxide layer. The removal of the silicon wafer up to the thin layer can be accomplished by various methods, such as precision grinding and polishing or etching down to an etch-stop layer introduced by epitaxial methods, by ion-implantation of boron or carbon as disclosed by Goesele and Lehmann in U.S. Pat. No. 5,024,723 or by other appropriate methods. All of these methods have in common the result that the removed substrate is lost.
Bruel, in U.S. Pat. No. 5,374,564, disclosed a process for the production of thin semiconductor material films that is based on hydrogen implantation before bonding and subsequent heating after bonding, this heating after bonding being required to be at temperatures higher than the temperature at which the hydrogen is implanted. According to Bruel's patent this process leads to the transfer of a thin semiconductor layer, the thickness of which is defined by the maximum in the concentration of implanted hydrogen. The layer transfer process was also described by Bruel in U.S. Pat. No. 5,374,215 and also in his paper entitled "Silicon on insulator material technology", which was published in Electronic Letters. in volume 31 in 1995 on pages 1201 to 1202 and in the paper by L. Di Cioccio, Y. Le Tiec, F. Letertre, C. Jaussaud and M. Bruel entitled "Silicon carbide on insulator formation using the Smart Cut process", which they published in Electronic Letters, in volume 32 in 1996 on pages 1144 to 1145.
Thin, monocrystalline silicon layers on an oxidized silicon substrate, thin monocrystalline silicon layers on a glass substrate with a thermal expansion coefficient close to that of silicon and thin monocrystalline silicon carbide layers on a glass or on oxidized silicon substrate have been realized by wafer bonding of hydrogen implanted monocrystalline silicon substrates and a subsequent heat treatment which causes the separation of a thin layer from the first substrate by the formation, growth and coalescence of hydrogen filled microcracks essentially parallel to the bonding interface and final macroscopic splitting at a location close to the maximum in the concentration of the implanted hydrogen whereby the monocrystalline thin layer is transferred to the second substrate. However, only in cases where only a small or no difference in the thermal expansion coefficients exists between the first substrates (as, for example, silicon or silicon carbide wafers) and the second substrates to which they are to be bonded is the method described by Bruel possible. Other material combinations such as silicon (as the first substrate ) and fused quartz (as the second substrate) suffer fracture from thermal stresses due to thermal mismatch between the two dissimilar substrates during the required transfer heat-treatment thermal splitting step. Once the thin layer is separated or split off the hydrogen implanted first substrate, a subsequent high temperature treatment would be allowed as long as the layer thickness is below a critical value, which depends on the specific material combination. For example, in the case of silicon on quartz, or gallium arsenide on silicon, or indium phosphide on silicon, the thermal expansion coefficient of silicon is 2.6.times.10.sup.-6 /.degree. C. while that of synthetic quartz is only 0.5.times.10.sup.-6 /.degree. C., that of gallium arsenide is 6.8.times.10.sup.-6 /.degree. C. and that of indium phosphide is 4.8.times.10.sup.-6 /.degree. C. at room temperature. A wafer bonded 4"-diameter standard silicon/quartz pair (both of .about.525 .mu.m in thickness), is found to crack at temperatures as low as 220 degrees centigrade. Since with monocrystalline silicon as the hydrogen implanted first substrate, the transfer heat-treatment temperature in the process disclosed by Bruel typically is above approximately 500 degrees centigrade, the hydrogen implanted silicon substrate in the bonded silicon/quartz pair has to be thinned down to less than about 150 .mu.m to avoid cracking of the bonded pair during the layer-splitting heat-treatment. In this case a main advantage of the process disclosed by Bruel is lost, because extensive lapping and etching to remove most of the first substrate is needed. In the method of the present invention the first substrate, from which the Si layer is transferred, can be reused. The high splitting temperature required for the thin layer transfer in the process disclosed by Bruel practically prevents the economical application of the method to transfer monocrystalline thin layers onto a dissimilar substrate with a substantially different thermal expansion coefficient.
Hydrogen induced microcracks are not all at exactly the same depth and are distributed around the maximum in the concentration of implanted hydrogen. Therefore, a certain roughness after the layer transfer results, which limits the thickness uniformity of the transferred film and requires an additional polishing step. The remaining surface roughness is especially difficult to remove for very hard materials, such as silicon carbide for which the layer transfer by hydrogen implantation induced microcracks has also been shown to work. The transfer of thin monocrystalline layers of very expensive materials such as silicon carbide to appropriate less expensive substrates such as polycrystalline silicon carbide is especially interesting for economic reasons since then one monocrystalline substrate can be used to fabricate many mononocrystalline layers of the same area as the original substrate. This process is only economical if the required polishing of the surfaces after the hydrogen microcrack induced layer transfer is not extensive. Another disadvantage associated with the layer transfer method induced by hydrogen implantation as presently practiced is the generation of extensive damage regions, which are still present after the layer transfer process and which have to be annealed out at a much higher temperatures after the layer-splitting process.
In the case of the transfer of thin monocrystalline layers from one monocrystalline substrate to another substrate having an equal or similar coefficient of thermal expansion, a method for performing the final splitting process at a low temperature is needed if at least one of the substrates contains temperature sensitive structures. A sufficiently low splitting temperature in combination with appropriate other processing steps leads to a reduced microroughness of the surface of the transferred layer due to a smaller depth distribution of hydrogen induced microcracks or to less hydrogen implantation induced damage remaining after the layer transfer.
In the case of the transfer of a monocrystalline layer to a substrate of very different thermal expansion coefficient as, for example, in the transfer of a monocrystalline silicon layer to a fused quartz glass substrate for application in flat panel displays, it is desirable to restrict the temperature necessary for the process of splitting by the overlap of microcracks to as low a temperature as possible and to avoid as much implantation induced damage as possible.